Make some people nervous, irritate others, cause chaos, force a shift of resources, close down the transport system, bring a city to a standstill, work towards crippling an economy... and demonstrate that small groups of people with very limited money and means have the wherewithal to strike when and how they please.
Among other things, a city is made up of complex networks and systems, organic as well as technological, social as well as physical. Such networks and systems have vulnerable intersections or nodes. Identify those and systems disruption is easy. Readable analyses by an expert on technology, terrorism, and global markets.